OmniDiskSweeper doesn't seem to show sizes for files in another user's folder. I suspect the other program is the same. Is there another user who may have a large iTunes library or something, or ripped DVDs etc? If so, you need to run the size checker while logged in as the other user.

On my machine, the amount Omni DS shows as free is less than the total minus the amount used. The difference is made up by about 2GB in my wife's iPhoto library.

OmniDiskSweeper doesn't seem to show sizes for files in another user's folder. I suspect the other program is the same. Is there another user who may have a large iTunes library or something, or ripped DVDs etc? If so, you need to run the size checker while logged in as the other user.

On my machine, the amount Omni DS shows as free is less than the total minus the amount used. The difference is made up by about 2GB in my wife's iPhoto library.

Yes that's right, it doesn't have permission to search other user's folders. The Finder doesn't either. My guess is your wife has a massive store of pornography. If she doesn't, put some in there and then confront her about it and see what she does.

I think there are also certain folders that Whatsize etc doesn't search. One of the most notable ones is /Volumes. This is where external drives are mounted. If you perhaps save stuff to an external drive or disk image, some programs create a folder in /Volumes and save stuff there.